This invention relates generally to phase locked loop frequency synthesizers and more particularly to a phase locked loop frequency synthesizer which utilizes a continuously adaptive loop filter which effectively has two smoothly joined slopes to its charge transfer characteristic, resulting in a fast mode of operation for large frequency variations and a slow mode of operation for small frequency variations. Such a synthesizer can simultaneously achieve fast lock time, low sideband noise, and low reference spurious levels, at low cost.
In traditional phase locked synthesizers, a tradeoff must be made between noise performance and the time required to achieve a given frequency offset from the final desired frequency. A frequency synthesizer which is used in electronic equipment requiring rapid periodic changes in frequency such as "frequency hopping" communications equipment is confronted with requirements such that a traditional loop cannot simultaneously achieve the needed lock-up time and noise performance. Proposed digital radiotelephone equipment, for example, may be required to frequency hop as far as 25 MHz in the approximately 1000 MHz band of operation and achieve an RMS phase error of less than 5 degrees in under 866 microseconds. This phase error also includes any receiver and detector errors. Therefore, the synthesizer must be very clean and very fast. It would be possible to meet both requirements by direct synthesis techniques, but these techniques are prohibitively expensive for a competitive cellular telephone subscriber unit.
One synthesizer circuit which at first glance has a similar appearance to the present inventive synthesizer is that shown and described in U.S. Pat. No. 4,387,348, "Phase-Controlled High Frequency Oscillator", issued to Bernd Fritze. This high frequency oscillator circuit is reproduced here as FIG. 1 and uses a correction current which is proportional to the integral of the difference of the short term charge pulses supplied by phase comparator charge pump current sources. That is, high frequency components and spikes which are output from the charge pump current sources are separated from the long duration, low frequency frequency control pulses and used to develop a correction current. The correction current is used to cancel long term drift of the high frequency oscillator caused by varactor diode leakage, by unequal residual currents in the current sources, and/or by other long term drift effects.
It is possible to make a different use of the high frequency components of the charge pump outputs. Proper connection of these high frequency signals can be used to great advantage in decreasing the lock time of a frequency synthesizer.